Various organic light emitting devices have been under active study and development, particularly those based on electroluminescence (EL) from small organic materials. For such organic devices, the ability to form morphologically stable amorphous films is a key requirement for the development of small materials for organic light emitting diodes (OLEDs). That is because when a small molecule compound is used as the organic light-emitting material, crystallization usually occurs if the molecule of the compound is too small and its structure is too symmetrical. Therefore, when applied in an organic light-emission layer, the small molecule compound is vulnerable to morphological change such as crystallization, and once the crystal is formed, it yields negative impacts upon the light-emitting nature and service life of the OLED.
Thermal stress during device operation can lead to such phase transitions from the amorphous state to the thermodynamically stable polycrystalline state leading to dramatic degradation of the device. As a result it is crucial to design materials featuring high glass transition temperature (Tg>150° C.) in order to stabilize the amorphous state. For improving the stability of devices in order to increase operational lifetime, several host materials have been reported.
Especially, designing materials having a spiro linkage has been a very successful strategy to obtain OLEDs materials with enhanced morphological stability while keeping their electro-optical functionality.
US2006/0141287 discloses light-emitting layers which include a solid organic material containing a mixture of at least two components. The first host component is an organic compound capable of transporting electrical charges and also forms an aggregate. The second component of the mixture is an organic compound capable of transporting electrical charges and, upon mixing with the first host component, is capable of forming a continuous and substantially pin-hole-free layer. In the reference, as the second component, various compounds such as substituted fluorene derivatives, and spirobifluorene derivatives, etc. are used.
US2010/0072887 also discloses light-emitting devices which are made of layers containing organoselenium compounds such as dibenzoselenophene, benzo[b]selenophene, or benzo[c]selenophene derivatives. These organoselenium compounds may serve as hosts for phosphorescent organometallic dopants.
In addition to the above patent references, there are several literature references disclosing spirobifluorene compounds. In Advanced Materials (Weinheim, Germany) (2004), 16(18), 1624-1629, hosts based on oligomers of the carbazole and 9,9′-spirobifluorene (spiro) building blocks, especially para and meta interconnected oligomers, suitable for efficient color-tunable triplet emission, are investigated in terms of the triplet excited-state properties. In this literature reference, the improvement of hole and electron injection in hosts for blue-, green- and red-light emission is also expected for the oligomer. Further, Chemical Physics Letters (2008), 461(1-3), 9-15 also includes simulation results on a series of spiro-linked oligofluorenes and derivatives.
JP 2010/027681 discloses 3,6-Bis-N-carbazolyl-9,9′spirobifluorene.
However, none of the above-disclosed materials meets all the requirements necessary for OLED application, particularly suitable energy level for high phosphorescent efficiency (high triplet energy), high morphological stability, while maintaining other electro-optic and processing properties under operational conditions of the device, such as emission color, dimensional stability, etc. Thus, there has been a need to develop new host materials, which are capable of satisfying all of the requirements indicated above.